Typical optical manipulation methods operate in fluidic media. We report a novel optical manipulation technique that enables optical manipulation in a pseudo-solid media, and can immobilize particles in 3D at any prescribed locations for reconfigurable colloidal assembly and tunable light-matter interactions.
The flourishing field of light-powered micro/nanorotors provides promising strategies for manufacturing and biomedical needs. However, the torque of optical rotors typically arises from the momentum exchange with photons, which limits the geometries and materials of objects that can be rotated and requires intense laser beams with designed intensity profile and polarization. These factors inhibit the light-powered rotation of highly symmetric or isotropic targets. Herein, we developed an optothermal micro/nanorotors platform that enables the rotation of various colloids with diverse sizes, materials, and various shapes, including live cells and micro/nanoparticles with high symmetry and isotropy. The long-sought-after out-of-plane rotation has been achieved by a single plane-polarized Gaussian laser beam with an ultralow power. This simple rotor approach is foreseen to open new horizons in colloidal and life sciences by offering a non-invasive and universal manipulation.
We present opto-thermoelectric speckle tweezers (OTEST) for large-scale and high-throughput trapping of particles. OTEST combine optical speckle with plasmonic substrate to generate a thermal speckle field that consists of many random thermal hotspots to trap a large number of particles using thermoelectric forces. We demonstrate trapping of dielectric and metallic particles with sizes as small as 100 nm in the speckle field. Finally, we integrate OTEST with microfluidic systems to demonstrate filtration of the smaller-sized particles from a mixed solution of 200 nm and 1 µm particles.
Several studies have been proposed to control particle trajectory in liquid solutions using optically induced thermal gradient. Upon introducing different solutes such as salts and surfactants along with microparticles in these solutions, an additional optically induced thermoelectric trapping force is generated due to the differential motion of ions in the solution under thermal field. As the complexity of the solution increases, it becomes increasing difficult to understand particle response towards laser irradiance. More importantly, the existing models to study the thermoelectric behavior of the particle assumes a constant temperature gradient across the particles, which becomes obsolete in the micro-regime due to discontinuity of thermal conductivity at the particle-solution interface. For a better understanding of trapping and manipulation behavior of particles under light induced thermoelectric field, the temperature gradient distortion must be considered. In this work, full-scale finiteelement solver model has been proposed to determine the temperature variation around a microparticle under laser heating. The resultant temperature distribution is utilized to numerically evaluate the thermoelectric field and the trapping potential of the laser induced opto-thermoelectric trap. To experimentally validate this methodology, polystyrene micro-particles are trapped opto-thermoelectric-ally in CTAC solution and compared the experimental trapping stiffness to theoretical estimates obtained from the model. It is observed that trapping stiffness saturates as surfactant concentration increases which can be optimized by choosing the lowest CTAC concentration at the onset of saturation. The model implemented here can be easily extended to arbitrarily shaped particles, particles with non-uniform surface morphology, different combinations of core-shell particles and electrolyte solutions, which can be implemented to study different phenomenon such as optical pulling, rotation and translation.
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